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What triggers a chemical reaction? - Kareem Jarrah

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Chemicals are in everything we see, and the reactions between them can look like anything from rust on a spoon to an explosion on your stovetop. But why do these reactions happen in the first place? Kareem Jarrah answers this question by examining the two underlying forces that drive both endothermic and exothermic chemical reactions: enthalpy and entropy.

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Meet The Creators

Educator Kareem Jarrah

Director Darcy Vorhees

Animator Tom Beuerlein

Script Editor Emma Bryce

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All chemical reactions involve the formation and breaking of bonds in chemicals in order to make new chemicals. Watch these beautiful chemical reactions and see what happens. The chemicals you start out with, called the reactants, have their bonds broken or form new bonds to make the products, the chemicals at the end of a reaction. But all of this bond breaking and bond making in chemicals either uses or releases energy.

Enthalpy, can you define it?Watch this Crash Course Chemistry lesson on it and find out more. If a chemical reaction gives off heat to the surroundings it is called an exothermic reaction. The chemical potential energy stored within the bonds of the chemical is given off during the reaction to give products that are lower in energy. The difference in energy is what you feel as heat radiating from the reaction, such as the burning of a log. On the other hand, if a chemical reaction needs to absorb energy from the surroundings it is called an endothermic reaction. Check out this experiment and see how an endothermic reaction can be produced. The products will have a higher chemical potential energy than the reactants so the reaction needs to draw energy from the surroundings to store it within the products. For example, ice packs become cool because of a chemical reaction that occurs after you crush them. This reaction draws the heat from the surroundings to power the formation of the products. This is felt as the ice pack becomes cold as it is drawing heat from your body to turn the reactants into products. Watch this TED-Ed video on the Chemistry of cold packs to find out more!.

Feeling in chaos because you want to know more about entropy? Watch this Crash Course Chemistry lesson and make some order out of that chaos! Entropy is a measure of the disorder a chemical has. The benchmark chemists use for 0 entropy (meaning no randomness) is a perfect crystal at absolute zero. What exactly is absolute zero? How close have scientists gotten to reaching this temperature? Read this article and find out the latest news! Anything else has at least some inherent disorder. However we can figure out relative entropy by looking at how different chemicals behave. A solid is contained and does not move freely, so it has low entropy. A liquid is much more free flowing so it has higher entropy. A gas is very free flowing, and hard to trap once released, so it has the highest entropy.

The second law of thermodynamics, cleaning your room, and the end of the universe

The second law of thermodynamics states that the entropy of the universe is always increasing, which means there is no chemical reaction you can do that will decrease the overall entropy of the universe. Visit this NASA site and get another explanation of this law. Surely, some things are becoming more ordered all the time. What if you clean your room, does THAT decrease the entropy of the universe? Well, not exactly. Let’s look at it this way: before you clean your room, you have a nice sandwich for lunch to increase your energy. That means that the energy needed to clean your room came from you ingesting food and breaking it down in your body. This increases the randomness of the universe. By cleaning your room, you may have made that little area of the universe more ordered, but the universe as a whole has more disorder. Maybe you can use this as a good excuse to not clean your room next time!

Sadly, there is another consequence to the second law of thermodynamics. If the entropy of the universe is always increasing what does that mean for the future of chemical reactions? Will there exist a day when the universe is at maximum entropy? What happens then? If the universe is at maximum entropy then chemical reactions will halt. There are two driving factors to chemical reactions, enthalpy and entropy. If entropy cannot be increased any further, chemical reactions cannot take place. This is called the Heat Death of the Universe Theory. It is the likely end to the universe. Interested in the death of the universe? Watch this TED-Ed lesson! Not to worry though, our universe still has a lot of order and it will be a long time-billions of years-until it has reached maximum disorder.

About TED-Ed Originals

TED-Ed Original lessons feature the words and ideas of educators brought to life by professional animators. Are you an educator or animator interested in creating a TED-Ed original? Nominate yourself here »